1. Field of the Invention
The invention is in the field of semiconductor manufacturing and more specifically in the field of manufacturing multilayer structures that include copper.
2. Related Art
Dielectric barrier layers including Cu—SiC or Cu—Si3N4 are commonly used in semiconductor devices. For example, these dielectric barrier layers may be incorporated within advanced back-end-of-line (BEOL) metallization structures. It has been found that the inclusion of a cobalt-alloy capping layer deposited between the copper layer and the SiC or Si3N4 layer results in improved adhesion between the layers and improved electro-migration and copper diffusion characteristics. The cobalt-alloy capping layer can be deposited on copper by chemical vapor deposition (CVD) or by electroless deposition.
Electroless deposition of cobalt alloys such as CoWBP or CoWP on copper has been demonstrated. A typical approach is to use a cobalt salt, a tungsten salt, a hypophosphite reducing agent, a borane reducing agent such as DMAB (dimethylaminoborane), and a complexing agent in a highly alkaline environment. For example, deposition usually occurs around a pH of 9 or above. When the cobalt alloy is to be used for adhesion improvement purposes only, the tungsten and phosphorus may be unnecessary as these elements are included principally to improve resistance to copper diffusion by stuffing the Co grain boundaries and reducing or eliminating Cu diffusion paths.
Electroless deposition can be inhibited by the presence of a thin copper-oxide layer on the copper. This copper-oxide layer forms when the copper is exposed to air or other oxidizing environment. Further, contaminants on the copper and dielectric surfaces can cause pattern-dependent plating effects such as pattern-dependent variations in the thickness of the cobalt-alloy capping layer. There is, therefore, a need to limit the formation of native copper oxide on the copper layer prior to deposition of the cobalt-alloy capping layer. Typically, the processing environment is controlled to limit this oxide formation, and also to remove any copper oxide and organic contaminants already on the copper surface. Unfortunately, the use of highly alkaline solutions in the electroless deposition of cobalt alloys, as in the prior art, promotes rather than limits the formation of copper oxides.